The present invention relates to spray guns in general, and in particular to a pneumatically assisted hydraulic atomizing spray gun having improved air/fluid turn on timing.
Pneumatically assisted hydraulic atomizing apparatus for paint and other liquids is well known and has gained acceptance in the spray coating industry. Such an apparatus is often referred to as an air-assisted airless spray gun, and effects atomization of coating liquids through both pneumatic and hydraulic means. With such an apparatus, liquid coating material is hydraulically atomized by delivering the material under a relatively high hydrostatic pressure to and through a small selectively shaped orifice that forms the emitted liquid into a fan-shaped coherent film that atomizes into a spray at its forward end. Although the pressure of the coating liquid delivered to the orifice is sufficient to effect its atomization, it is less than sufficient to achieve a degree of atomization that is satisfactory for high quality coating purposes. Consequently, to improve the atomization, such air-assisted airless apparatus also includes means for directing one or more streams of air against the coherent film of coating liquid emitted from the orifice. The energy of the air is transferred to the film, with the result that the energy available for effecting atomization is increased and the degree of atomization is improved.
Air-assisted airless spray guns have fluid and air valves for controlling flows of coating liquid to the orifice and of air to an air nozzle. In accordance with conventional practice, the turn on timing of the fluid and air valves is controlled so that when the gun is turned on the air valve is opened before the fluid valve, and when the gun is turned off the fluid valve is closed before the air valve. However, the inventors have observed that with such an air-assisted airless spray gun, dried paint may be "spit" off of its fluid tip and onto product being painted, and have found that the cause of such "spitting" is attributable to the conventional air/fluid valve timing sequence. ln particular, it has been found that the source of the dried paint was from a build-up of paint on the exterior of a fluid tip defining the selectively shaped orifice, which in turn resulted from air syphoning liquid coating material out of a cavity in the fluid tip behind the orifice and between the orifice and fluid valve. The syphoning effect occurs when the gun is triggered, because with a conventional gun the air comes on before and goes off after the fluid. Frequent triggering of the gun results in a build-up of paint on the tip, which eventually flakes off and is carried by the air and/or coating material to the article being painted.